In recent years, a nonvolatile semiconductor memory with a structure that has a particle layer including conductive particles, such as Si nanocrystals, inserted in a tunnel oxide film has been developed as an advanced MONOS nonvolatile semiconductor memory (e.g., refer to JP-A 2003-78050 [KOKAI]). The structure of this memory is such that charges can go in and out in the form of tunnel current between the Si surface and the trap level in the Si nitride film (charge storage layer) via a double tunnel junction that has Si nanocrystals meeting Coulomb blockade conditions sandwiched between tunnel oxide films.
In retaining memory, the tunnel of information charge is blocked by an energy barrier ΔE caused by quantum confinement and the Coulomb blockade effect of Si nanocrystals. Therefore, the memory retention characteristic can be improved exponentially according to exp(ΔE/kBT). In a write/erase operation, a suitable write/erase voltage is applied, allowing information electrons to tunnel through without being affected by the energy barrier ΔE. Consequently, high-speed write/erase operation can be performed.
To miniaturize this type of semiconductor memory, it is necessary to make each layer thinner. However, it is necessary to make a block insulating film sufficiently thick to prevent stored charges in the charge storage layer from being discharged to the gate electrode, causing the problem of making more difficult the thinning of a block insulating film than a tunnel insulating film. In addition, use of a multilevel configuration is effective in increasing capacity. However, a memory using particles has the problem of being difficult to realize a multilevel configuration.
A nonvolatile semiconductor memory which causes particles themselves to store charges has been proposed (e.g., refer to JP-A 2003-318293 [KOKAI]). However, this type of semiconductor memory requires the particle diameter and variance of particles to be precisely controlled to cause particles themselves to store charges, which causes the problem of low reliability.